Interspecies platelet immunizations in marmosets lead to formation of antibody to the donor platelets, deposition of IgG on the host's platelets and thrombocytopenia. The clinical and physiologic manifestations of this disease closely resemble that observed in posttransfusion purpura in man, a hematologic disorder falling within the syndrome of idiopathic thrombocytopenic purpura. The form of platelet (antigen) administration and route of injection are important variables since platelets suspended in saline and inoculated intravenously are without effect while intramuscular injections frequently lead to the disease syndrome. This differentiation in response to route of administration is due to the nature (molecular species) and titer of antibody formed. Antibody toward autologous or host-type platelets have also been found, but these are detectable only when the animal is severely thrombocytopenic. Immune reagents obtained during the thrombocytopenic period have led to discrimination of platelet antigens within two species of marmosets. This in turn now permits intraspecies immunizations for induction of the disease comparable to that found in humans. Tests for a direct cytopathic effect of cells towards platelets have indicated this may be one mechanism whereby platelet homeostasis may be altered in animals experiencing immune thrombocytopenia.